1. Field of the Invention
This invention relates to an image-forming method for forming images having good gloss characteristics and image quality, in printers and copying machines employing a system in which toner images formed by electrophotography are transferred to recording mediums and also the toner images are fixed to the recording medium.
2. Related Background Art
In general, it is well known that silver salt photography and printing have wide color reproduction space and good graininess, gloss uniformity and so forth and hence have a superior image quality. On the other hand, color images formed by electrophotography, though having remarkably been improved in image quality in virtue of digitization in recent years, have not yet achieved so high image quality as the silver salt photography and printing have done.
In full-color images, the gloss characteristics of images contribute greatly to the quality of images. In the silver salt photography and printing, sharp images which are smooth and free of any gloss non-uniformity are obtained by means of the smoothness of paper which is a recording medium, and a color-developing agent of submicrons in particle diameter.
In dry-process electrophotography, in general, electrostatic latent images are formed by means of exposure light on a photosensitive member which is an image-bearing member, the electrostatic latent images are developed with toners of yellow, magenta, cyan and black colors (prepared by mixing colorants such as pigments or dyes in thermoplastic resins) to form toner images, and then the toner images are electrostatically transferred to a recording medium, followed by melt-fixing by applying heat and pressure to form a color (or full-color) image.
In such dry-process electrophotography, usually, toners having particle diameters of from 3 xcexcm to 12 xcexcm are used, and it is common to form secondary colors and tertiary colors by the use of four color toners of yellow, magenta, cyan and black which have such particle diameters. Hence, a toner or toners of one color to three colors or four colors are used depending on a color or colors to be reproduced, so that the quantity in which the toner(s) is/are laid on the recording medium (i.e., toner laid-on quantity) may differ part by part. The unevenness of image surface has a tendency of coming small in proportion to the toner laid-on quantity (density) on the recording medium. Hence, where the toner laid-on quantity stands different over the whole image, its glossiness may come non-uniform.
For this reason, in images having a large density gradation as in portrait images, the images have a low gloss at halftone areas (low-density regions) like those of skin and a high gloss at solid areas (high-density regions) like those of hair, giving non-uniform gloss of image and hence a sense of incongruity.
It is also known that any unevenness the surface of such images may have makes them have a low color reproducibility under the influence of irregular reflection from the image surface to provide images having a low sharpness.
Under such circumstances, as disclosed in, e.g., Japanese Patent Application Laid-Open No. 63-92965, a technique is proposed in which, on a transfer material such as paper provided with a transparent resin layer of 50 to 100 xcexcm thick, color toners with average particle diameter of, e.g., 10 to 15 xcexcm are superimposed in two to four layers to form color toner images, which are then heated by means of a heat roll to make the color toners melt in the transparent resin layer, followed by fixing to form a color image. As also similarly disclosed in Japanese Patent Application Laid-Open No. 3-38659, a technique is proposed in which, on a transfer material such as paper provided with a transparent resin layer of 10 to 500 xcexcm thick, and preferably 25 to 300 xcexcm thick and using color toners with average particle diameter of, e.g., 8 xcexcm, color toner images are formed, which are then fixed by means of a heat roll to form a color image.
In the techniques disclosed in the above publications, when color toner images are fixed onto a transfer material, the color toner images are pressed by means of a heat roll to heat and melt them and so fix them as to be buried in the transparent resin layer at the transfer material surface so that color images with less surface unevenness can be formed and any illumination light may less irregularly reflect to obtain color images having good quality.
However, in the above method, an oil film is formed between the toners and the transparent resin at the time of fixing, under the influence of a silicone oil which is a release agent, so that the toner images are not sufficiently buried in the transparent resin layer and hence some unevenness remains inevitably.
As also disclosed-in Japanese Patent Application Laid-Open No. 5-216322, a method is proposed in which toner images are electrostatically transferred to a recording medium provided on its surface with a transparent resin layer formed of a thermoplastic resin in a thickness of 20 to 200 xcexcm, and thereafter the toner images are buried in the transparent resin layer by means of a belt type fixing assembly. In this method, however, even if the toner images and the thermoplastic transparent resin layer are sufficiently melted, they are not sufficiently compatible with each other when melted. Hence, this may cause a lowering of color reproducibility, and may make the image surface remain a little uneven to cause color non-uniformity or narrow the region of color reproduction. This method has such problems.
To solve such problems, Japanese Patent Application Laid-Open No. 11-84719, for example, discloses that images free of any image non-uniformity and having a stable gloss can be obtained by specifying toner laid-on quantity, fixed-image density and image glossiness. This enables formation of high-grade images with less non-uniform gloss in the low-gloss region, but on the other hand has not achieved formation of sufficiently high-grade images in the high-gloss region. Moreover, because of a great difference in glossiness from that of the recording medium itself, there is a problem it gives a sense of incongruity.
The present invention was made taking account of the above problems. Accordingly, an object of the present invention is to provide an image-forming method which promises a uniform image gloss without relying on the toner laid-on quantity on the recording medium, and can keep images from giving any sense of incongruity between the image gloss and the recording-medium gloss.
To achieve the above object, the present invention provides an image-forming method having at least:
a developing step of developing an electrostatic latent image held on an image-bearing member, by means of a toner to obtain a toner image;
a transfer step of transferring the toner image to a recording medium; and
a fixing step of heating the toner image having been transferred onto the recording medium, to fix the former to the latter; and
having at least:
a low-speed mode (PS1) in which images are formed at a low speed; and
a high-speed mode (PS2) in which images are formed at a high speed;
so as to be able to form images at different speeds;
the recording medium having a gloss value (60-degree gloss) represented by G0 and the toner being laid on the recording medium in a quantity ranging from 0.05 mg/cm2 or more to 1.3 mg/cm2 or less, under conditions of which;
(i) when a recording medium having a high gloss (G0 greater than 40) is used, an image is formed at a low-speed mode (PS1) in which the process speed is from 20 mm/sec. or more to less than 130 mm/sec. and, where the maximum value and minimum value of gloss value of the image having been fixed are represented by Gmax1 and Gmin1, respectively, the Gmax1 and the Gmin1 satisfy the following expressions:
Gmax1xe2x89xa6G0+40 and Gmin1xe2x89xa7G0xe2x88x9225; and
(ii) when a recording medium having a low gloss (0xe2x89xa6G0xe2x89xa640) is used, an image is formed at a high-speed mode (PS2) in which the process speed is from 130 mm/sec. or more to 600 mm/sec. or less and, where the maximum value and minimum value of gloss value of the image having been fixed are represented by Gmax2 and Gmin2, respectively, the Gmax2 and the Gmin2 satisfy the following expressions:
Gmax2xe2x89xa6G0+20 and Gmin2xe2x89xa7G0xe2x88x9215.
The present invention also provides an image-forming method having at least:
a developing step of developing an electrostatic latent image held on an image-bearing member, by means of a toner to obtain a toner image;
a transfer step of transferring the toner image to a recording medium; and
a fixing step of heating the toner image having been transferred onto the recording medium, to fix the former to the latter; and
having at least:
a low-speed mode (PS1) in which images are formed at a low speed; and
a high-speed mode (PS2) in which images are formed at a high speed;
so as to be able to form images at different speeds;
in the fixing step, the relationship between fixing temperature (T1) in the low-speed mode and fixing temperature (T2) in the high-speed mode being:
T2 less than T1.